Since it seems we're going ahead with this one, I would expect it to be tied up quite quickly. Rennes already bought Ludo Blas and have Doué (another great young talent) to fill the gap on the right wing. They've been building for his departure (and a departure of Lovro Majer who is still there for now) for a while. And they don't tend to play hardball with transfers tbh. This is the club that let Ousmane Dembele go for peanuts, after all.
Rennes are technically one of the richest clubs in the world. Owned by the Artemis group of Francois Pinault, who, last time I checked, was among the top 30 richest men on the planet. They have no issues with money but he barely invests anything, they just rely on the academy to make stars and sell them on. It's worked pretty well for the most part.